Multiple versions of MKL dlls exist in the install directory of Solidworks 2021. Indeed, the dlls supporting FloXpress and simulation seem to be the updated MKL version that no longer support the flag. However, the main executable only seems to call sldmkl_parts.dll. It appears to be MKL version 2018.1.156 that does support the flag
It would depend on the version of MKL. If Solidworks has (just for example) statically linked to or bundled in an old version of MKL, then it should work there, still.